Bacterial kidney disease (BKD) results from infection by Renibacterium salmoninarum. BKD is a chronic and systemic disease that generally leads to mortality in juvenile and adult salmonids, both in fresh water and marine environments. Bacterial Kidney Disease of Salmonid Fish, Annu. Rev. Microbiol., 35:273-298. Salmonids are fish of the family Salmonidae, which are soft-finned fishes such as salmon, trout, chars and whitefishes.
Renibacterium salmoninarum is a slow-growing gram-positive bacterium. The bacterium is endemic in wild anadromous (migrating up rivers from the sea to breed in fresh water) salmonid populations on both coasts of North America, and has been found in wild Atlantic salmon and sea trout. After infection, Renibacterium salmoninarum localizes in the kidney from which infection rapidly becomes systemic.
Farming of marine species is an ancient practice, and aqua culturing of fish has increased significantly over the last twenty years. Commercial aqua culturing requires maintaining high densities of cultured fish. This increases the likelihood of economic loss from diseases such as BKD relative to less-dense fish populations. Although the actual losses attributed to BKD have not yet been calculated, the disease is known to be one of the most important bacterial diseases affecting resident anadromous salmonid stocks in the Pacific Northwest. Because BKD is one of the most prevalent diseases of cultured salmonids, it has had a significant economic impact on the fishing and aqua culture industries.
There still are limited effective methods for controlling BKD despite its economic impact. One reason for this is that the bacteria is capable of adjusting to different conditions as an intracellular parasite, and has the ability to survive and multiply in phagocytic cells (cells that engulf and digest foreign bodies). Current approaches to managing BKD outbreaks include stress reduction, quarantine, chemotherapy (antibiotic treatment), total destruction of the infected population and complete sterilization of the facilities. These approaches to BKD infection are not commercially appealing, and are difficult to administer to large fish populations.
There are no known vaccines effective for treating fish susceptible to infection by BKD, despite the continuous efforts by those skilled in the art. For instance, McCarthy reported an attempt to vaccinate fish susceptible to BKD using two preparations of formalin-inactivated cells of Renibacterium salmoninarum. McCarthy et al., Immunization of Rainbow Trout, Salmo gairdneri, Against Bacterial Kidney Disease: Preliminary Efficacy Evaluation, J. of Fish Dis., 7:65-71 (1984). The bacterins were administered without adjuvant by IP-injection, immersion, or two-step hyperosmotic infiltration. No significant protection was afforded by these methods.
Furthermore, Kaattari et al. have treated salmonids with a number of potential immunogens in an attempt to confer immunity to fish susceptible to BKD infection. These immunogens included cell-wall fractions, fractured cells and extracellular products. Kaattari et al., Development of a Vaccine for Bacterial Kidney Disease, Bonneville Power Administration Final Report, (1990). These immunogens were administered by intraperitoneal injection, orally, and by immersion with and without adjuvant. None of these early preparations protected fish. In fact, some of these preparations exacerbated the disease.
The route of delivering vaccines often is an important factor for the successful vaccination of fish. Intraperitoneal vaccination is generally the most effective method for vaccinating any species, even though IP vaccination is labor intensive. Immersion is another vaccination method, which is widely used on smaller fish (fish that weigh less than about 10 to 15 grams). The standard immersion method involves exposing fish to the vaccine in aerated standing water for a minimum of 20 seconds. The disadvantage of immersion vaccination is that it is limited by the weight of fish that can be immunized per unit volume of vaccine. And, immersion vaccination usually provides lower levels of immunity than other techniques, due to the stress it causes fish.